Geologic Time. Kentucky Geological Survey Modified McCann

Transcription

1 Geologic Time Kentucky Geological Survey Modified McCann

2 Procedure Plate tectonics Classroom activity #1 What is geologic time? Relative age dating Absolute age dating Classroom activity #2 T C

3 What is time? A partitioned perspective of our own existence Years, months, days, weeks, hours, etc. Events that have happened over the years How to tell time geologically Using the geologic time scale

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6 Geologic Time and Earth s Age Early scholars (pre 18th-19th century) written in biblical scriptures cooling rates of magma/earth depositional rates of sedimentary rocks calculating age of oceans through salinity James Hutton founder of modern geology ( ) Principle of uniformitarianism the same processes today have operated over vast amounts of time Charles Lyell (1830) Principles of Geology Championed Hutton s uniformitarianism in more detail The present is the key to the past

7 Putting events in order Television common in homes Wright brothers fly at Kitty Hawk First World War American bicentennial Walking on the moon Television common in homes Second World War First World War Wright brothers fly at Kitty Hawk s

8 Relative age dating Which rocks or events are older? Which are younger? Rocks and events to be placed in sequential order. Obvious, but discovery was a very important scientific achievement in interpreting geologic history Nicholas Steno ( ) Danish anatomist credited with 6 Principle of Relative Dating Superposition Original horizontality Lateral continuity Cross-cutting relationships Inclusions Fossil succession

9 Principle of superposition In an undisturbed sequence of sedimentary rock, oldest layers are at the bottom and the youngest are at the top Basis of relative-age determination A B

10 Principle of original horizontality Sediment is deposited in essentially horizontal layers Therefore, a sequence of rock at an angle from horizontal must have been tilted after deposition and lithofication

11 Principle of lateral continuity layers of sediment extend laterally in all directions until it thins or terminates against and adjacent basin

12 Principle of cross-cutting relationships Igneous intrusions or faults must be younger than the rocks they intrude

13 Principle of inclusions Inclusions or fragments of rock contained within a layer of another are older than the rock layer itself

14 Principle of fossil succession William Smith ( ) Canal builder Fossils at the bottom of a sequence of rock strata are older than those at the top. Specific groups of fossils follow, or succeed, on another in a definite order. Relative ages can be determined How is it recognized? Life has varied through time (~95% of all organisms are extinct!!!) Identified fossil assemblages are recognizably different from one another Identifying index fossils

15 Composite Hill Quarry Stream

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17 Unconformities Surfaces representing periods of non-deposition, erosion, or both A gap in the geologic record A book with missing pages!! 3 types Disconformity Angular unconformity Nonconformity

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19 Applying principles of relative age dating

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21 Absolute age dating Based on radioactive decay of certain isotopes Variable forms of the same element Produces an actual #, powerful tool to date geologic events Discovered in 1903 by Pierre and Marie Curie

22 Absolute age dating Process by which an unstable atomic nucleus is transformed into an atomic nucleus of a different element Called radioactive decay Decay rates are discussed in terms of a half-life time it takes for half of the atoms of the original parent element to decay to new atoms of a daughter element Half-lives are constant

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24 By measuring the parent-daughter ratio and knowing the half-life of the parent, geologists can calculate the age of a rock sample containing the radioactive element Done in laboratory with a mass spectrometer. Expensive!!

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26 Absolute age dating Crystallization of mineral v.s. time of formation of atoms Not for sedimentary rocks Metamorphic overprinting

27 Combining relative and absolute age dating